The Kawasaki Ki-78 was a high-speed research aircraft that failed to live up to expectations, but that did introduce a number of features new to the Japanese aircraft industry.
The aircraft began life as a civil research project, the KEN III (Kensan III or Research III), at the University of Tokyo, in 1938. Work on the design progressed slowly, and a wooden mock-up wasn't completed until May 1941. Kawasaki were then brought into the project and asked to produce two prototypes, only one of which was completed. Work on this first prototype began in September 1941, but after the Japanese entry into the war the project was taken over by the Army, and the design was given the specification Ki-78.
The Ki-78 was powered by an imported Daimler-Benz DB 601A 12-cylinder liquid cooled engine. From the side the aircraft had a similar profile to most aircraft that used this engine, with a long nose and slightly low-mounted propeller (although the cock-pit was almost flush to the fuselage), but from the front its small dimensions became apparent. The fuselage was designed to have the smallest cross section possible (similar to the approach taken by Messerschmitt on the Bf 109), and the small size of the wings also became apparent. The Ki-78 had a wingspan of 26ft 2 31/32in and a wing area of 118.4 sq ft. In contrast the Bf 109 had a wingspan of 32ft 4 1/2in and a wing area of 174 sq ft, and yet was still originally seen as having a small wing.
Engine cooling was provided by two radiators mounted on the rear fuselage, each with a small air intact sticking slightly out from the fuselage. Further cooling was provided by a fan driven by a small turbine. The engine provide 1,175hp normally, or 1,550hp with water-methanol injection (one of the features new to the Japanese aircraft industry). The small wings had a laminar flow section, and were given Fowler flaps, split flaps and drooping ailerons in an attempt to lower the landing speed.
The prototype made its maiden flight on 26 December 1942. It was heavier than expected, increasing the wing loading. The landing speed was also higher than expected, at 106mph, and the aircraft were very difficult to fly at lower speeds. Its performance was also disappointing. The original aim had been to reach 850km/hr (528mph), but the aircraft never got anywhere near this speed. Its best performance came on its 31st test flight, on 27 December 1943, almost exactly one year after the maiden flight. Even then it could only reach 434.9mph. One more test flight was conducted, on 11 January 1944, and after that work on the project was suspended.
Engine: Daimler Benz DB 601A
Power: 1,175hp normally, 1,550hp with boost
Wing span: 25ft 2 31/32in
Length: 26ft 7in
Height: 10ft 7/8in
Empty Weight: 4,255lb
Loaded Weight: 5,071lb
Max Speed: 435mph at 11,485ft
Range: 373 miles